Flush mounted backfire circularly polarized antenna

ABSTRACT

A flush mounted backfire antenna and array receptive to circularly polarized radiation and particularly adapted for aircraft application. Two orthogonally phased elements such as half-wave slots are arranged so that the projected angle of the E field of the elements is 90* in a backfire direction.

United States Patent 1 Scherer FLUSH MOUNTED BACKFIRE CIRCULARLYPOLARIZED ANTENNA James P. Scherer, Sunnyvale, Calif.

[ 51 Nov. 27, 1973 7/1972 Van Atta 343/770 3/1959 Butler 343/770 PrimaryExaminer--Eli Lieberman Attorney-Alvin E. Hendricson et a1.

[57] ABSTRACT A flush mounted backfire antenna and array receptive tocircularly polarized radiation and particularly adapted for aircraftapplication. Two orthogonally phased elements such as half-wave slotsare arranged so that the projected angle of the E field of the elementsis 90 in a backfire direction.

7 Claims, 6 Drawing Figures [22] Filed: Apr. 27, 1972 [21] Appl. No.:248,110

[52] US. Cl. 343/770, 343/853 [51] Int. Cl. H0lq 13/10 [58] Field ofSearch 343/767, 770, 771,

[56] References Cited UNITED STATES PATENTS 2,812,514 11/1957 Smith343/767 PATENTEnRuvzvms 5,775,771

sum 2 or z 47 FIG. 4

FIG. 6

1 FLUSH MOUNTED BACKFIRE CIRCULARLY POLARIZED ANTENNA BACKGROUND OFINVENTION Antennas and antenna arrays incorporating slot radiators forpropagating and receiving circularly polarized radiation have long beenknown in the art. US. Pat. No. 2,767,395 discloses a beacon antenna forcircularly polarized radiation and a generalized discussion ofcircularly polarized slot radiators is set forth in the articleCircularly Polarized Slot Radiators by AJ. Simmons, IRE TRANSACTIONS ONANTENNAS AND PROPAGATION, Vol. AP-5, No. 1, January, 1957, at pages 31et seq. A further advance in this field is set forth in U.S. Pat. No.2,982,960 wherein slot radiators are provided in a rectangular waveguideto achieve an arbitrarily polarized slot radiator.

There have also been developed a wide variety of flush mounted antennaswhich are paricularly adapted to aircraft use and other applicationswherein space requirements may be exacting. Flush mounted antennas orradiators are commonly linearly polarized. Examples of flush mountedantennas which radiate in the backfire direction but are linearlypolarized are to be found in an article entitled A New Class of LogPeriodic Antennas by J.W. Greiser, pages 617 and 618 of May, 1964PROCEEDINGS OF THE IEEE and an article entitled A Log Periodic CavityBack Slot Array by Antoine Georges Roederer, appearing at pages 756 to758 of the November, 1968 IEEE TRANSACTIONS ON ANTENNA AND PROPAGATION.Attempts to produce circularly polarized flush mounted antennas havebeen limited generally to broadside antennas and antenna arrays.

The present invention is particularly directed to the provision of aflush mounted antenna of simple and relatively inexpensive constructioncapable of propagating and receiving circularly polarized radiation inthe backfire direction.

SUMMARY OF INVENTION The antenna of the present invention comprises aflush mounted structure requiring only a minimal backing cavity so as tobe particularly applicable to aircraft and the like. In accordanceherewith a slot radiator is comprised of a pair of half-wave slotsarranged so that the center lines thereof are crossed at an obtuse anglein the backfire or endfire direction of the radiatorand the phasecenters of the slots are coincident. The coincident phase centersproduce circularly polarized radiation over a wide range of angles. Theslots are energized in 90 phase relationship and in order to maintainthis relationship the slots themselves do not actually cross or toucheach other, but instead, each slot is formed as two slot portions. Theangle between the radiating elements or slots of the present inventionis set to establish the projected angle of the E field of the twoelements equal to 90 along the center line of the radiator.

The antenna of the present invention may be simply constructed byetching radiation slots in a printed circuit board disposed over a smallbacking cavity and energizing the slots 90 out of phase with microstripor coaxial feed lines, for example. Half-wave coaxial lines may beemployed to couple opposite portions of each slot at the center of thecrossed radiating elements. A

total antenna depth of about 1 inch is attained hereby to thus achieve atruly flush antenna structure It will be appreciated that antennacapabilities are in part measured by frequency sensitivity and withregard to the present invention it is also important that response be atleast substantial over a wide range of angles. A single radiator inaccordance with the present invention has been found to provide an axialratio less than 3 decibels at 55 off broadside and a relatively goodaxial ratio at 30 off broadside in the forward fire direction whilemaintaining VSWR well below 2:1 at design frequency. Theoretically, asingle antenna element in accordance with this invention radiatescircularly polarized energy in the backfire and forward fire directionand periodic or log-periodic arrays are employed to couple the elementsonly to a backfire mode. Such an array is employed to maximize bandwidthand gain.

DESCRIPTION OF FIGURES The present invention is described as to a singlepreferred embodiment thereof in the accompanying drawings wherein:

FIGS; 1 and 2 are diagrammatic representations of slot orientation,field vectors and relevant angles;

FIG. 3 is a plan view of an antenna formed in accordance with thepresent invention;

FIG. 4 is a transverse sectional view taken in the plane 4-4 of FIG. 3and illustrating an internal arrangement of the antenna of FIG. 3;

FIG. 5 is a sectional view taken in the plane 5-5 of FIG. .4 andillustrating electrical connection of radiating slots and portionsthereof; and

FIG. 6 illustrates an antennaarray and energization in accordance withthe present invention.

DESCRIPTION OF PREFERRED EMBODIMENT Reference is first made to FIGS. 1and 2 of the drawings illustrating at 11 a ground plane having a pair ofslot elements 12 and 13 disposed therein. In FIGS. 1 and 2 the numbers12 and 13 actually represent the center lines of slot elements asfurther described below; however, for convenience, the term slotelement" or element is employed in the following theoretical discussion.The slot elements 12 and 13 will be seen to be disposed in crossingrelation at the centers of each, with such centers disposed on a centerline 14 of the antenna. The slot elements 12 and 13 are oriented in skewrelation to each other with each of the elements making an angle 4) withthe center line 14. The angle between the slot elements at the centerline 14, i.e., the angle 2, is greater than The direction of the E fieldis illustrated in FIG. 1 by the E field vectors 16 and 17. These Efieldvectors will be seen to be normal to the adjacent portions of theradiators 12 and 13 and thus to each make an angle a with the centerline 14. It thus follows that qb a 90.

FIG. 2 illustrates in perspective the ground plane 11 with the slotelements 12 and 13 thereon and E field vectors 16 and 17 being shown.The line or vector 18 is intended to represent the direction of circularpolarization from the antenna. The circular polarization line 18 isshown to make an angle of 6 with the ground plane 11. There is alsoillustrated in.FIG. 2 the angle ,3 which is the apparent projectionangle of the circularly polarized radiation and which is herein equal to90. From a mathematical consideration of the angles illustrated in FIGS.1 and 2, it will be seen that in general sin tangent a/tangent 8/Consequently, the angle of circularly polarized radiation from theground plane of the radiators is, in fact, related to the slot angle d)and thus the present antenna does operate to radiate or receivecircularly polarized radiation at a substantial angle to broadside,i.e., does comprise a backfire circularly polarized antenna.

One preferred physical configuration of the present invention isillustrated in FIGS. 3 to 5 wherein the same numerals 12 and 13 areemployed to identify the radiators or slot members. In order to precludeproblems arising from direct coupling between the radiators 12 and 13that would result from actual crossing or touching of the slots, each ofthe radiators is formed as two longitudinally aligned slots. Thus, theradiator 12 is shown to be comprised as a pair of longitudinally Ialigned slots 21 and 22 of equal length, with a slight spacing betweenadjacent ends thereof. Similarly, the radiator or slot element 13 iscomprised as a pair of longitudinally aligned slots 23 and 24 of thesame length and spaced apart at the adjacent ends thereof. The slotmembers or, more properly, the center lines 12 and 13 thereof will beseen to intersect at the centers thereof as described above. The slots21 and 22 are energized as described below to be in phase, andsimilarly, the slots 23 and 24 are energized to be in phase, so that thephase center of the slot pair 2122 coincides with the phase center ofthe slot pair 23-24.

The slot radiators may be formed by etching a copper layer 31 on adielectric plate 32 with such combination of dielectric and metal beingcomprised, for example, of a board for printed circuits. Energization ofthe radiators may be accomplished by the provision of microstrips 33 and34 conventionally formed of a thin dielectric strip 36 having a metalconductor 37 plated on one side thereof. These microstrips may bedirectly connected to the metal side of the plate 32 with the stripelectrode 37 separated from the copper layer 31 by the dielectric 36.Energization may be provided by a pair of microstrips, as illustrated,coupled, for example, to the slot portions 22 and 24 as shown. Placementof an energized microstrip across a slot will excite an electric fieldin a slot which, in turn, radiates. It is additionally noted thatplacement of the microstrip on a slot is such that the impedance of themicrostrip matches the impedance of the slot at the crossing, so thatmaximum energy is transferred from the microstrip to the slot. In thisrespect, reference is again made to the aboveidentified publication ofAG. Roederer for a discussion of strip placement with respect to a slotfor maximum coupling. It is also noted that the radiators may beenergized from coaxial cables, and in this instance such cables mighthave the outer conductor connected as by soldering to the ground plane,with a gap in the outer conductor where the inner conductor thereofcrosses the slot to be energized. Maximum coupling for coaxialenergization is also attained by appropriate placement of the coax withrespect to the slot in the same manner as microstrip coupling.

Coupling of the energized slot portions to the other portions of therespective slot pairs may be accomplished by the provision of coaxialconnectors 41 and 42, as illustrated in FIG. 5. Each of the coaxialconnectors is formed as a conventional cable of one-half wavelength atthe operating or nominal frequency of the antenna. Considering theconnector 42, for example, it

will be seen that the central conductor thereof is coupled across theslot portions 21 and 22 of the radiation element 12. Thus the two slotportions 21 and 22 are electrically coupled together in phase forenergization from the microstrip 33. Similarly, the slot portions 23 and24 of radiator 13 are coupled together in phase by the coaxial line 41for energization from the microstrip 34. Maximum coupling is provided byappropriate location of cable ends and slots in the same manner asdiscussed in the Roederer reference.

Each of the slot portions is formed as one-half wavelength slots at theoperating or nominal frequency of the antenna and the two radiators 12and 13 are energized out-of-phase. This energization may beaccomplished, for example, by providing a simple T- power splitter andan extra one-quarter wavelength in one of the microstrip lines or,alternatively, as described in connection with FIG. 6 below. This 90energization or phase difference between radiator energization isaccomplished in order to provide for the radiation of a rotating vectorto thus establish the circular polarization of radiation from theradiators. The sense or direction of the circular polarization may bedetermined by inserting the extra one-quarter wavelength in theappropriate line.

The antenna structure may be completed by the provision of a shallowrectangular metal housing 46 having an open top into which there fitsthe dielectric plate 32 and elements carried thereby. This housing 46has an open top to receive the upper portion of the antenna and may besubstantially filled with an absorbent material 47, as illustrated.

It will be appreciated that the slots of the present antenna radiatewhen the electric field thereof is perpendicular to the slot length andthus it will be seen that the E field vector is, in fact, normal to theslot length to establish the E field vector directions as indicated inFIG. 1. It will be appreciated that the dielectric plate 32 and metallicground plane 31 may be sealed to the open topped receptacle 46 to thusform a shallow sealed unit comprising a flush mounted circularlypolarized backfire antenna. As previously noted the total depth of thisantenna need only be about 1 inch for radiation in the microwave range.It is to be further appreciated that the above brief discussion ofantenna radiation properties is equally applicable to antenna receptionproperties and thus the antenna of the present invention is particularlyadapted to receive circularly polarized radiation from the back end ofthe antenna without undue attenuation thereof.

It is further noted that the antenna of the present invention may beprovided in a periodic array or logperiodic array. In FIG. 6 there isgenerally illustrated a periodic array 61 of antenna elements 62-65 inaccordance with the present invention. Another manner of energizing theantennas is illustrated in FIG. 6 as in cluding a 90 hybrid 71 havingports 72 and 73 and providing not only a power split between the outputsbut also a 90 phase difference therebetween. Microstrip or coaxial lines76 and 77 are illustrated to extend from the hybrid 71 into couplingrelationship with radiators of the antennas, and the lines 76 and 77being terminated by matching loads 78 and 79, respectively. In order forthe projected angle of the E fields from the successive pairs ofradiating elements to attain space orthogonally, there is to be provideda slight phase difference between the energization of successive pairsof elements. This phase difference may be provided by the length ofmicrostrip or coaxial line between successive radiators to which it iscoupled. The degree or amount of phase difference depends upon thephysical spacing of the successive pairs and in this respect referenceis made to the publication Directive Frequency Independent Arrays, byK.K. Mei et al., appearing in the September, 1965 IEEE TRANSACTIONS ONAN- TENNAS AND PROPAGATION regarding calculation of phase delay forcoupling to the backward wave mode.

There will be seen to have been described above a simple flush antennastructure capable of radiating or receiving circularly polarizedradiation in the backfire direction. It is furthermore noted that thedimensional tolerances of the elements of the invention are notcritical. There is provided hereby a truly practical advance in the art.It is not intended to limit this invention to the precise terms ofdescription or details of illustration for it will be apparent to thoseskilled in the art that numerous modifications and variations may bemade.

What is claimed is:

1. An antenna structure comprising electrically conducting meansdefining a ground plane having a center line longitudinally thereof,said means having a pair of crossed slot radiators therein with thecenter lines of said slots crossing each other at said center line andsymmetrically disposed with an angle greater than 90 therebetween atsaid center line, and means electrically energizing said slots ninetydegrees out-of-phase to thus radiate from the slots circularly polarizedradiation at an angle to broad side from said ground plane. 2. Theantenna of claim 1 further defined by each of said slots making an angleof d) with the center line of said ground plane, the E field vectors ofsaid slots mak- 6 ing an angle a -d) with the center line of said groundplane and the center of said circularly polarized radiation making anangle with the ground plane of 6 wherein sine tana/tan45.

3. The antenna of claim 1 further defined by said ground plane beingcomprised as a metal layer on a dielectric plate and the meansenergizing said slots including microwave transmission lines coupled toseparate slots.

4. The antenna of claim 1 further defined by means defining a shallowcavity disposed behind said ground plane.

5. The antenna of claim 1 further defined by each of said slots beingcomprised of two longitudinally aligned half-wave slot portions slightlyseparated at adjacent ends, and a half-wave transmission line couplingthe portions of each slot.

6. An antenna structure comprising an electrically conducting groundplane having at least two slot radiators therein,

said slot radiators having the center lines thereof crossing at thecenters of the radiators and having an angle between the center linesgreater than ninety degrees,

each of said slot radiators comprising a pair of longitudinal alignedslots spaced apart at adjacent ends with the phase centers of the tworadiators coincident, and

means electrically energizing said slot radiators ninety degrees out ofphase for propagating circularly polarized radiation from the antennastructure.

7. The antenna of claim 6 further defined by a plurality of pairs ofsaid slot radiators aligned on said ground plane to radiate in thebackfire mode.

1. An antenna structure comprising electrically conducting meansdefining a ground plane having a center line longitudinally thereof,said means having a pair of crossed slot radiators therein with thecenter lines of said slots crossing each other at said center line andsymmetrically disposed with an angle greater than 90* therebetween atsaid center line, and means electrically energizing said slots ninetydegrees out-ofphase to thus radiate from the slots circularly polarizedradiation at an angle to broadside from said ground plane.
 2. Theantenna of claim 1 further defined by each of said slots making an angleof phi with the center line of said ground plane, the E field vectors ofsaid slots making an angle Alpha 90*- phi with the center line of saidground plane and the center of said circularly polarized radiationmaking an angle with the ground plane of theta wherein sin theta tanAlpha /tan45*.
 3. The antenna of claim 1 further defined by said groundplane being comprised as a metal layer on a dielectric plate and themeans energizing said slots including microwave transmission linescoupled to separate slots.
 4. The antenna of claim 1 further defined bymeans defining a shallow cavity disposed behind said ground plane. 5.The antenna of claim 1 further defined by each of said slots beingcomprised of two longitudinally aligned half-wave slot portions slightlyseparated at adjacent ends, and a half-wave transmission line couplingthe portions of each slot.
 6. An antenna structure comprising anelectrically conducting ground plane having at least two slot radiatorstherein, said slot radiators having the center lines thereof crossing atthe centers of the radiators and having an angle between the centerlines greater than ninety degrees, each of said slot radiatorscomprising a pair of longitudinal aligned slots spaced apart at adjacentends with the phase centers of the two radiators coincident, and meanselectrically energizing said slot radiators ninety degrees out of phasefor propagating circularly polarized radiation from the antennastructure.
 7. The antenna of claim 6 further defined by a plurality ofpairs of said slot radiators aligned on said ground plane to radiate inthe backfire mode.